1. Field of the Invention
The present invention relates generally to a source of hydraulic fluid under pressure and more specifically to a source of hydraulic fluid under pressure which is particularly suited for use with an active suspension for the control of vehicular roll and pitch.
2. Description of the Prior Art
JP-A-63-251313 discloses an arrangement wherein a variable displacement type pump arrangement is operatively connected with a source of rotational energy, such as a prime mover (e.g. internal combustion engine) of an automotive vehicle, and the output of the pump is supplied to an active automotive vehicle suspension. In this arrangement vertical, lateral and fore/aft accelerations are used to determine the change in vehicle posture. When the change in posture exceeds a predetermined level, the amount of discharge from the pump to the active suspension is increased above a level used when the vehicle is stationary, and the increased discharge is supplied (by way of example) to a hydraulic fluid cylinder operatively connected between the sprung and unsprung components of the vehicle suspension.
However, this arrangement has suffered from a problem in that even though it aims to take advantage of the fact that the amount of power which is consumed when the vehicle is stationary, is less than that when the vehicle is running, the amount of pump discharge is always increased in response to large changes in vehicle posture. As the pump output is controlled on the high side during both high and low speed operations the pump load is undesirably increased under a number of operational modes when it is actually not needed and exerts a detrimental effect on the vehicle fuel economy.
For example, substantial fuel is consumed by the pump when the vehicle is running at very low speeds along an undulating road and the relative displacement between the chassis and the road wheels assumes a relatively large value. However, as the time frame in which the displacement occurs exhibits a very low ratio, corrective boosting of the pump displacement is not necessary and only results in fuel being wasted.